A conventional I.S. glass container forming machine comprises a series of sections arranged along side each other and operating out of phase with each other. Each section is equipped with a single parison transfer mechanism carrying one or more neck-rings. Taking single gob operation as an example, the machine cycle commences with the neck-ring in a reverted position and the parison mould closed around it. A charge of glass enters the upper end of the parison mould and is blown or pressed into a parison shape with some glass being forced into the space between the neck-ring and a plunger to form a finish end of the final container. The parison mould then opens leaving the parison held by its finish end in the neck-ring. The invert mechanism then operates to carry the parison from the parison forming station and to invert it into the blow mould station where the blow mould closes around it. The neck-ring is then opened, dropping the parison into the blow mould where it is supported by a bead formed on the finish. The invert mechanism returns the neck-ring to the parison forming station and the parison mould closes around it ready for the next charge of glass. Meanwhile the parison hanging in the blow mould is blown into the final container.
Various steps have been taken in recent years to increase the speed of the container forming process e.g. by the introduction of axial mould cooling methods and the reduction in the wall thickness of the finished container. However, the thickness of the glass in the finish has not been reduced to the same extent and in lightweight containers today the thickest glass is often to be found here. As machine speeds have increased, the time available for the forming and cooling of the glass in the finish has become disproportionately reduced. This is a particular problem when the parison is formed by a plunger in a press process, for the neck-ring is the last part of the parison mould equipment to be filled with glass. The result is that the glass in the finish is not cool enough, and thus not hard enough to maintain its shape accurately when the parison is transferred to the blow mould, leading to subsequent distortion and loss of ware. Further reduction in the weight of the finish is limited because, should the distance between the neck-ring and the plunger inside it become too small it is not possible to force glass into the neck-ring and containers are formed with incomplete finishes.
One method available to reduce this problem is to apply more cooling to the neck-ring. This is only partially effective because, if the neck-ring becomes too cold, defects are formed in the container finish. Also because of the very low thermal conductivity of the glass a cooler neck-ring does not take out appreciably more heat from the glass. A more effective solution to the problem would be to increase the time of contact between the finish of the container and the neck-ring but this is not possible with a single neck-ring per blank mould without slowing down the machine cycle which is commercially unacceptable.
The provision of two sets of neck-rings for each parison mould will overcome the problem described above, and has other benefits. The parison would be made and inverted in the normal way but instead of being dropped into the blow mould when the neck-rings open, the neck-rings would remain closed during at least part of the blowing operation. The extra time available for contact between the glass finish and the neck-rings would extend to almost a complete machine cycle if required. More time would also be available for forming the parison, for next cycle would not have to wait for the neck-rings to invert, open and return. Instead, an empty set of neck-rings would be available to move into position as soon as the filled neck-rings had left. Additionally, the speed of invert could be beneficially reduced, for the inverting set of neck-rings would not be required to be back in the reverted position as quickly as possible ready for the next cycle.
Several attempts have been made to devise ways of providing two sets of neck-rings on an individual section glass container forming machine. The main difficulty is interference between the set of neck-rings carrying the parison to the blow mould and the other, empty, set of neck-rings returning to the parison forming position.
One suggestion is made in U.S. Pat. No. 4,162,911, in which two sets of neck-ring mechanisms are provided one on either side of a parison mould assembly, and operate alternately to transfer parisons outwards to two blow mould assemblies also on opposite sides of the parison mould assembly. Such an arrangement leads to a machine wherein the sections have to be arranged transversely next to each other rather than parallel to each other, with consequent problems of space and glass supply.
U.S. Pat. No. 3,434,820 describes a machine in which a chain carrying a series of neck-rings travels in a vertical loop thus holding returning neck-rings clear of ones carrying glass. Such an arrangement is only usable in a machine in which the parison is formed in an upright rather than an inverted position.
U.S. Pat. No. 4,058,388 describes a machine with two neck-rings mounted on racks in which interference is avoided by sliding the two mechanisms horizontally parallel to the centre line of the section. Such an arrangement is mechanically complex and essentially requires a considerable width of the section.
It is one of the objects of the present invention to provide a parison transfer mechanism which enables two sets of neck-rings to be provided in a conventional I.S. Section to work in co-operation with one set of parison moulds.